In the production of chemical pulp, lignin and other organic non-cellulosic material is separated from the raw material of chemical pulp by cooking using cooking chemicals. Cooking liquor used in chemical digestion, i.e. waste liquor, is recovered. The waste liquor, which is separated mechanically from the chemical pulp, has a high combustion value due to carbonaceous and other organic, combustible material contained therein and separated from the chemical pulp. The waste liquor also contains inorganic chemicals, which do not react in chemical digestion. Several different methods have been developed for recovering heat and chemicals from waste liquor.
Black liquor obtained in sulfate pulp production is combusted in a recovery boiler. As the organic and carbonaceous materials contained in black liquor burn, inorganic components in the waste liquor are converted into chemicals, which can be recycled and further utilized in the cooking process.
Hot flue gases are generated in black liquor combustion, which are led into contact with various heat exchangers of the recovery boiler. Flue gas conveys heat into water or vapor, or a mixture of water and vapor, flowing inside the heat exchangers, simultaneously cooling itself. Usually flue gases contain abundantly of ash. Main part of the ash is sodium sulfate, and the next largest part is usually sodium—carbonate. Ash contains other components, too. The ash entrained in flue gases is in the furnace mainly in vaporized form, and starts to convert into fine dust or smelt droplets mainly in the parts of the boiler downstream of the furnace. The salts contained in the ash melt, or they are sticky particles even at relatively low temperatures. Molten and sticky particles stick easily onto heat transfer surfaces and even corrode them. Deposits of sticky ash have caused a clogging risk of the flue gas ducts, and also corrosion and wearing of the heat surfaces in the boiler.
A chemical recovery boiler is conventionally formed of the following main parts, which are illustrated schematically in FIG. 1:
The furnace of a recovery boiler comprises a front wall and side walls. The width of the furnace refers to the horizontal length of the front wall and the depth refers to the length of the side wall of the furnace. FIG. 1 illustrates the structure of a chemical recovery boiler having a furnace defined by water tube walls, a front wall 11, side walls 9 and a rear wall 10, and also a bottom 15 formed of water tubes. Combustion air is fed into the furnace from multiple different levels 18. The air levels can be located also differently from what is presented in the Figure. Waste liquor, such as black liquor, is fed into the furnace from nozzles 12. During combustion, a smelt bed is formed onto the bottom of the furnace. The smelt is removed from the bottom of the furnace via a conduit 17, typically via smelt spouts.                A lower part 1 of the furnace, where combustion of waste liquor mainly takes place.        A middle part 2 of the furnace, where the final combustion of gaseous combustible substances mainly takes place.        An upper part 3 of the furnace        A superheater zone 4, wherein the saturated steam exiting the steam drum 7 is converted into (superheated) steam having a higher temperature. In the superheater zone or in front of it there is often a so-called screen tube surface or screen tubes, which usually acts as a water reboiler.        in a flue gas duct following the furnace are the heat transfer exchangers downstream of the superheaters: a boiler bank and economizers, wherein the heat of flue gas generated in the furnace is recovered. The boiler bank 5, i.e. water vaporizer, is located in the first flue gas channel of the flue gas duct, i.e. in a so-called second pass. In the boiler bank the water in saturated temperature is partly boiled into vapor.        Feed water preheaters, i.e. so-called economizers 6a, 6b, wherein the feed water flowing in the heat transfer elements is preheated by means of flue gases prior to leading the water into the drum 7 and into the steam-generating parts (boiler bank 5, walls of the furnace and possible screen tubes) and into superheating parts 4 of the boiler.        A drum (or steam drum) 7 having water in the lower part and saturated steam in the upper part. Some boilers have two drums: a steam drum (upper drum) and a water drum (lower drum), between which a heat exchanger, so-called boiler bank tubes for boiling the water are provided.        Other parts and devices in conjunction with the boiler, such as e.g. a combustion air system, a flue gas system, a liquor feeding system, a treatment system for smelt and liquor, feed water pumps etc. A so-called nose is marked with reference numeral 13.        
The water/steam circulation of the boiler is arranged via natural circulation, whereby the water/steam mixture formed in the water tubes of the walls and bottom of the furnace rises upwards via collection tubes into the steam drum 7 that is located crosswise in relation to the boiler, i.e. parallel to the front wall 11. Hot water flows from the steam drum via downcomers 14 into a manifold of the bottom 15, where from the water is distributed into the bottom water tubes and further into the water tube walls.
The preheater i.e. economizer typically refers to a heat exchanger comprising heat transfer elements, inside which the boiler feed water to be heated flows. Free space for flue gas flow remains in the economizer between the heat exchanger elements. As the flue gas flows by the heat exchanger elements, heat is transferred into the feed water flowing inside the elements. The boiler bank is also formed of heat transfer elements, inside which the water to be boiled or a mixture of water and steam flows, into which the heat is transferred from the flue gas flowing past the elements.
The heat exchangers for heat recovery, i.e. boiler bank and economizers, are usually constructed so that in them the flue gas flows not from down upwards, but usually only from above downwards. In economizers, the flow direction of water is usually opposite to the flow direction of flue gases in order to provide a more economical heat recovery.
In some waste liquor recovery boilers the boiler bank is constructed so that the flue gases flow substantially horizontally. In single drum boilers having such a horizontal flow boiler bank, the heat transfer elements of the boiler bank are positioned so that the water to be boiled flows substantially from down upwards. The boiler bank here is referred to as a horizontal flow boiler bank because the flue gases flow substantially horizontally. Two drum boilers are typically provided with an upper drum and a lower drum, between which the boiler bank tubes are located so that the water to be boiled flows in the tubes substantially from down upwards, and the flue gases flow substantially horizontally. In these cases, a common term cross flow can be used for the flue gas and water streams, or a term cross flow boiler bank for the boiler bank.
In a conventional waste liquor recovery boiler illustrated schematically in FIG. 1, which has a so-called vertical flow boiler bank 5, the flue gases flow vertically from above downwards. A flow channel 8 for flue gases is arranged adjacent to the boiler bank, in which channel the flue gases that have flown through the boiler bank 5 flow from down upwards. The channel 8 is as conventional devoid of heat exchangers. Next to the channel 8 there is a first economizer (a so-called hotter economizer) 6a, wherein the flue gases flow from above downwards, transferring heat into the feed water that flows in the heat exchanger elements of the economizer. In a corresponding way, a second flue gas channel 9 is arranged next to the hotter economizer, in which channel the flue gases coming from the lower end of the economizer 6a flow upwards. Also this flue gas channel is, as conventional, a substantially empty channel without heat exchange elements for heat recovery or water preheaters. Next to the flue gas channel 9 is a second economizer, a so-called colder economizer 6b, in which the flue gases flow from above downwards, heating the feed water flowing in the heat exchange elements.
In addition to the boiler bank 5, two economizers 6a and 6b and the channels 8, 9 between them, the boiler can have several corresponding flue gas channels and economizers.
As is known, the flue gases in the boiler bank and in the economizers are arranged to flow from above downwards. The ash entrained in the flue gases fouls the heat transfer surfaces. As ash particles stick onto the heat transfer surfaces, the ash layer gradually gets thicker, which impairs heat transfer. If ash accumulates abundantly on the surfaces, the flow resistance of the flue gas can grow into a disturbing level. Heat transfer surfaces are cleaned with steam blowers, via which steam is from time to time blown onto the heat transfer surfaces, whereby the ash accumulated onto the surfaces is made to come loose and pass with the flue gases into ash collection hoppers located in the lower part of the heat transfer surface.
Not all recovery boilers are provided with a boiler bank. European patent application 1188986 presents a solution, in which the first flue gas duct part downstream of the recovery boiler, the so-called second pass, is provided with at least one superheater, especially a primary superheater. Then a problem can be excess increase of the temperatures of surfaces in this part of the flue gas duct. WO patent application 2014044911 presents that said part of the flue gas duct is arranged for being cooled with cooling medium coming from the screen tubes.
European patent 1728919 presents an arrangement, where the part of the flue gas duct downstream of the recovery boiler, the so-called second pass, is provided with both a boiler bank and an economizer one after the other in the incoming direction of the flue gas, but the superheater surfaces are located, corresponding to prior art, in the upper part of the furnace of the boiler. When the second pass is provided with a boiler bank and an economizer, it limits the positioning of other heat surfaces, such as a superheater surface, in the flue gas flow.
There are also solutions, in which the electricity production of a chemical pulp mill is suggested to be improved by means of a reheater located in the recovery boiler. The reheater and the superheater are in principle and in practice similar heat transfer surfaces. A difference is that in “actual” superheaters (which in this patent application is called a superheater) saturated steam exiting a boiler drum is superheated step by step to a hotter temperature (e.g. to a temperature of approximately 515° C.), until after the last step it is called live steam. The live steam is then led into a steam turbine for production of electrical energy. In a reheater, in its turn, steam obtained from a turbine is heated and after that returned back into the turbine. Bled steams are taken from the turbine at predetermined pressure levels and they are used e.g. for heating the feed water or combustion airs. When using a reheater, the steam remaining in the turbine is led at an optimized pressure back into the boiler, into a reheater, where the steam is heated and the heated steam is taken back into the turbine for improving the production of electricity. In known solutions, such as in U.S. Pat. Nos. 7,640,750 and 8,443,606, reheaters are located in a conventional superheater zone in the upper part of the furnace. However, this kind of arrangement decreases the space for superheaters or the height of the boiler and thus the whole boiler building has to be increased. U.S. Pat. No. 7,640,750 presents a two-stage reheater, the latter stage of which is located in a cavity in the recovery boiler. Fuel is combusted in the cavity for producing flue gases.